Millions of Americans already have high cholesterol in their 20s and 30s — and don’t know it. Here’s why that worries doctors.

It’s estimated that millions of Americans are walking around with dangerous levels of “bad” cholesterol in their blood well before their 40th birthday. But many have no idea, because doctors typically don’t screen, much less treat, people for high cholesterol in their 20s or 30s. U.S. guidelines don’t call for Americans to be screened during young adulthood, but some doctors think that ought to change, considering that high cholesterol rates are rising, and the condition is a leading cause of heart attacks and strokes.

Here’s why screening starts later in life, and why some cardiologists are going off-book.

First, what is considered high cholesterol?

When your doctor says your cholesterol is “high,” that typically refers to your combined blood levels of “good” high-density lipoprotein (HDL) cholesterol and “bad” low-density lipoprotein (LDL) cholesterol being over 200 milligrams per deciliter (mg/dL). But it’s high levels of LDL — 160 mg/dL or higher — cholesterol that make that number worrisome. It’s important that your combined cholesterol number stays relatively low, and that your HDL levels are higher than your LDL levels, because “good” cholesterol helps to remove “bad” artery-hardening cholesterol, according to Medline.

Levels are considered healthy if:

• Total cholesterol is under 200

• LDL cholesterol is under 100

• HDL cholesterol is 60 or higher

You’re considered at-risk or borderline high if:

• Total cholesterol is between 200 and 239

• LDL cholesterol is between 100 and 159

• HDL cholesterol is between 40 and 59 for men, or 50 and 59 for women

And levels are high if:

• Total cholesterol is 240 or higher

• LDL cholesterol is 160 or higher

• HDL cholesterol is under 40 for men, or under 50 for women

Scientists haven’t studied cholesterol much in young people

In general, doctors test for signs of health problems, then decide the best course of treatment.

But doctors sort of work backward when it comes to high cholesterol in young adults, Dr. Andrew Moran, a Columbia University professor of general medicine whose research focuses on the prevention of cardiovascular disease, tells Yahoo Life.

Treating high cholesterol with medication isn’t recommended by the U.S. Preventive Services Task Force (USPSTF) for anyone under 40. In fact, the USPSTF’s recommendations say that cholesterol-lowering drugs should be given only to patients between the ages of 40 and 75 if they have at least a 7.5% risk of heart disease in the next 10 years. The widely used heart disease risk calculator won’t even calculate the heart disease risks for someone younger than 40. And the USPSTF no longer has a separate guideline for screening without treatment. “So, a lot of [health care providers] think, ‘Why should I screen for high cholesterol if there’s nothing to do about it?’” in terms of a prescription, Moran says.

The most commonly prescribed drugs to treat cholesterol are statins. Doctors prescribe them because statins have been widely studied and shown to be safe and effective in large, gold-standard clinical trials — but there haven’t been many of those trials involving young adults.

In part, that’s because it would be expensive and slow to test whether statins work for young people. The important measure for a clinical trial of statins is whether people using them have fewer heart attacks and strokes than those not taking the drugs. “To conduct a clinical trial, if you have a limited budget, the smart thing to do is to find the people who are relatively high-risk, because in a relatively short amount of time, you’ll be able to measure the difference,” explains Moran. “With young people, it” — a major cardiovascular event — “probably won’t happen for 20 or 30 years into the future.”

So, if scientists haven’t proven that statins work and are safe for younger people, U.S. health agencies won’t recommend the drugs and doctors won’t prescribe them. And if there’s nothing to prescribe to lower a young person’s cholesterol, then the guidelines suggest there’s little reason to screen for it.

But some cardiologists think that’s incorrect.

Lowering cholesterol early could delay or prevent heart disease

Driven in part by American diets, many young adults already have high or borderline-high cholesterol in their 20s and 30s. The Centers for Disease Control and Prevention estimates that 7.5% of Americans between ages 20 and 39 have high total cholesterol (greater than 240 mg/dL). Moran’s research suggests that as many as 27% of young adults have high levels of “bad” LDL cholesterol (the kind responsible for plaques and heart attacks), but don’t yet have heart disease. And some 40% of Americans of any age don’t know what their cholesterol level is, according to a recent study published in JAMA Cardiology.

Cholesterol accumulates in the bloodstream over a lifetime, beginning as early as childhood. So, the earlier you can act, the better. High cholesterol can certainly be managed and treated later in life, but that “would not have the [same] impact had they started in their 30s. It’s easier to get [cholesterol] lower the younger you are,” Dr. Muhammad Siyab Panhwar, an interventional cardiologist at Sanford Health, tells Yahoo Life.

If someone’s cholesterol is screened when they are in their 20s or 30s, there’s a good chance that lifestyle changes — like a healthier diet, more exercise, less alcohol and quitting smoking — will be enough to get their levels under control. Limiting saturated fats found in red meat and dairy alone can reduce bad cholesterol levels by as much as 10%, according to the Mayo Clinic.

These changes are always the first choice in managing young adults’ cholesterol, Panhwar says. But “if you have done all of the [non-medication] stuff and are still struggling with high cholesterol, you need to consider medication,” he says. “There is no other option. There is no supplement that some internet health and wellness guru will sell you that can lower it.”

There are some valid concerns about the side effects of statins, says Moran. Taking them long-term may raise the risk of developing type 2 diabetes. But for millions of Americans with high cholesterol, “​​the research done on this topic shows the benefit from preventing heart attack and stroke outweighs the risk of diabetes,” Moran says.

But the first, and most important, step is screening in the first place. Even though it’s not recommended by the guidelines, Moran and Panhwar advise young adults to ask their doctors to check their cholesterol, especially if they have a family history of high cholesterol or heart problems. “If I have a 30-year-old with high cholesterol in my clinic, I am not waiting 10 years until they turn 40 and then use a calculator to calculate a neat little score for them,” says Panhwar. “That’s silly. They need management now, not after 10 years of unchecked high cholesterol.”